1. Field of the Invention
The present invention relates to fiber amplified spontaneous emission (ASE) light sources, and more particularly, to superfluorescent fiber sources that have a mean wavelength that is stable with respect to changes in pump polarization and fiber birefringence.
2. Description of the Related Art
Fiber ASE light sources are well known in the art. ASE sources have been advantageously used to provide wideband (e.g., on the order of 10 to 30 nanometers), spatially coherent light for multiple applications. For example, ASE sources have been used to provide laser light as an input to a fiberoptic gyroscope. For a description of an exemplary superfluorescent fiber source, see an article entitled xe2x80x9cAmplification of Spontaneous Emission in Erbium-Doped Single-Mode Fibersxe2x80x9d by Emmanuel Desurvire and J. R. Simpson, published by IEEE, in xe2x80x9cJournal of Lightwave Technology,xe2x80x9d Vol. 7, No. 5, May 1989.
An ASE light source typically comprises a length of single-mode fiber, with a portion of its cross-section (typically the core) doped with an ionic, trivalent rare-earth element. For example, neodymium (Nd3+) and erbium (Er3+) are rare-earth elements that may be used to dope the core of a single-mode fiber so that it acts as a laser medium.
The fiber receives a pump input signal at one end. The pump signal is typically a laser signal having a relatively narrow spectrum centered around a wavelength xcexp. The ions within the fiber core absorb the input laser radiation at wavelength xcexp so that electrons in the ground state of these ions are excited to a higher energy state of the ions. When a sufficient pump power is input into the end of the fiber, a population inversion is created (i.e., more electrons within the ions are in the excited state than are in the lower laser state), and a significant amount of fluorescence is generated along the length of the fiber. As is well known, the fluorescence (i.e., the emission of photons at a different wavelength xcexs) is due to the spontaneous return of electrons from the excited state to the lower laser state so that a photon at a wavelength xcexs is emitted during the transition from the excited state to the ground state. These photons are amplified by the gain as they travel down the fiber, leading to amplified spontaneous emission (ASE). The light which is emitted at the wavelength xcexs from the fiber is highly directional light, as in conventional laser light. However, one main characteristic of this emission which makes it different from that of a traditional laser (i.e., one which incorporates an optical resonator) is that the spectral content of the light emitted from the superfluorescent fiber source is generally very broad (typically several tens of nanometers). This principle is well known in laser physics, and has been studied experimentally and theoretically in silica-based fibers doped with erbium, neodymium, or other rare earths, for several years.
Light emitted from ASE fiber sources has multiple applications. For example, in one application, the output of the ASE source is fed into a fiberoptic gyroscope. For reasons that are well understood by those skilled in the art, the fiberoptic gyroscope should be operated with a broadband source which has a highly stable mean wavelength. Of the several types of broadband sources known to exist, superfluorescent fiber sources, in particular, made with erbium-doped fiber, have been thus far the only optical sources which meet the stringent requirements for inertial navigation grade fiberoptic gyroscopes. The broad bandwidth of light produced by erbium-doped fiber sources, together with the low pump power requirements and excellent mean wavelength stability of erbium-doped fiber sources, are the primary reasons for use of such sources with fiberoptic gyroscopes.
In an erbium-doped fiber, the emission of a superfluorescent fiber source is bi-directional. That is, the light which is emitted by the return of electrons to the ground state in the erbium ions is typically emitted out of both ends of the fiber. As described in U.S. Pat. No. 5,185,749, to Kalman, et al., for erbium-doped fibers of sufficient length, the light propagated in the backward direction (i.e., in the direction opposite that in which the pump signal propagates) has a very high efficiency. Thus, it is advantageous to implement erbium-doped sources so that the light emitted from the ASE erbium-doped source is emitted from the pump input end of the fiber (i.e., in the backward propagation direction).
An ASE source is generally implemented in one of two configurations. In a first configuration, called a single-pass ASE source, the superfluorescent source output power is emitted in two directions, one of which is not used. In the second configuration, called a double-pass ASE source, a reflector is placed at one end of the doped fiber to reflect the superfluorescent source signal so that the superfluorescent signal is sent a second time through the fiber. Since the fiber exhibits gain at the superfluorescent signal wavelengths, the ASE signal is further amplified. One advantage of the double-pass configuration is that it produces a stronger signal. A double-pass ASE source configuration also produces output only at one port (i.e., in one direction). A disadvantage of such a configuration is that the feedback optical signal from the gyroscope must be kept very low in order to prevent lasing (e.g., with use of an optical isolator located between the source and the gyroscope).
For fiberoptic gyroscope applications, one critical measure of source performance is the stability of the source mean wavelength (for example, see U.S. Pat. No. 5,355,216 to Kim, et al.). As is well known in the art, stability of the source mean wavelength leads directly to the stability of the gyroscope scale factor. Precise knowledge of the scale factor is critical for an accurate measurement of the rotation rate of the gyroscope. Presently, superfluorescent fiber sources exist which have a mean wavelength stability with respect to pump power, pump wavelength, temperature, and level of optical feedback down to a few parts per million each, assuming reasonable stabilization of system parameters such as pump wavelength, pump power, temperature and optical feedback from the gyroscope. However, an overall stability of better than one part per million in mean wavelength is desirable for some applications, in particular, high-grade fiberoptic gyroscopes.
Polarization effects have recently been shown to play a role in the instability of the mean wavelength of superfluorescent fiber sources (SFS). The polarization dependence of the mean wavelength of an SFS output has been predicted through numerical modeling by J. L. Wagener, et al. [see J. L. Wagener, xe2x80x9cErbium doped fiber sources and amplifiers for optical sensors,xe2x80x9d Ph.D. thesis, Applied Physics Department, Stanford University (March 1996); and J. L. Wagener, M. J. F. Digonnet, and H. J. Shaw, xe2x80x9cA High-Stability Fiber Amplifier Source for the Fiber Optic Gyroscope,xe2x80x9d J. Lightwave Technol. Vol. 15, 1689-1694 (September 1997); and J. L. Wagener, D. G. Falquier, M. J. F. Digonnet, and H. J. Shaw, xe2x80x9cA Mueller Matrix Formalism for Modeling Polarization Effects in Erbium-Doped Fiber,xe2x80x9d J. Lightwave Technol. Vol. 16, 200-206 (February 1998) which are hereby incorporated by reference herein]. These studies have shown that the mean wavelength of the SFS depends slightly on pump polarization. The reason for this can be explained in physical terms as follows. The ions of erbium (or another dopant, such as Nd or another rare earth) in the fiber host experience an intrinsic anisotropy of absorption and emission with respect to polarization. For example, some erbium ions more strongly absorb a given polarization than others, and correspondingly, these erbium ions have a preferred polarization associated with their emission. This effect gives rise to polarization-dependent gain when the erbium-doped fiber is pumped in the usual manner, i.e., by a highly polarized source such as a laser diode. This in turn can result in orthogonal polarization components of the output ASE signal having different mean wavelengths.
One aspect of the present invention is a superfluorescent source that comprises an optical pump and a laser medium. The laser medium has a dopant therein that produces amplified spontaneous emission (ASE) whose spectrum has a mean wavelength that is different for different polarization components of the ASE. The medium is pumped by pump light from the optical pump. An optical coupler optically couples the pump to the medium. A Faraday rotator mirror reduces the polarization dependence of the output of the source. The mirror reflects ASE emitted from the medium back through the medium. An output port couples ASE from the source. In one embodiment, the source further comprises a depolarizer between the medium and the coupler. In one embodiment, the mirror is coupled to the optical coupler. In one embodiment, the mirror is coupled to the medium. In one embodiment, an optical isolator is positioned at the output port to reduce optical feedback. In one embodiment, a fiber optic gyroscope receives output from the superfluorescent source. In one embodiment, the medium is polarization maintaining and the pump light is directed into the medium in such a way that equal powers of the pump light are launched along birefringence axes of the medium. In one embodiment, the pump light is linearly polarized and is introduced to the medium at 45 degrees with respect to the birefringence axes of the medium. In one embodiment, the dopant is erbium. In one embodiment, the reduction of polarization dependent gain in the medium reduces the mean wavelength difference between the respective spectral outputs of orthogonal polarization components. In one embodiment, the reduction of the polarization dependent gain reduces any dependence of the mean wavelength of any polarization component on birefringence of the medium. In one embodiment, the mirror reflects unabsorbed pump light back through the medium to further reduce the polarization dependence of the output. In one embodiment, the medium includes an optical fiber. In one embodiment, the medium includes an optical waveguide.